This invention relates to the application of coatings in a vacuum apparatus. In particular, this invention relates to an apparatus which generates a plasma of electrically conducting materials for the application of coatings to surfaces of a substrate by way of condensation of plasma. The apparatus can be used in mechanical engineering, instrument and tool making, in the production of electronic equipment, and in other fields of industry.
Many types of vacuum arc coating apparatus utilize a cathodic arc source, in which an electric arc is formed between an anode and a cathode plate in a vacuum chamber. The arc generates a cathode spot on a target surface of the cathode, which evaporates the cathode material into the chamber. The cathodic evaporate disperses as a plasma within the chamber, and upon contact with the exposed surfaces of one or more substrates coats the substrates with the cathode material, which may be metal, ceramic, etc. An example of such an arc coating apparatus is described in U.S. Pat. No. 3,793,179 issued Feb. 19, 1974 to Sablev, which is incorporated herein by reference.
An undesirable result of the vacuum arc coating technique is the creation of macroparticles, which are formed from molten cathode material vaporized by the arc. These macroparticles are ejected from the surface of the cathode material, and can contaminate the coating as it is deposited on the substrate. The resulting coating may be pitted or irregular, which at best presents an aesthetic disadvantage, but is particularly problematic in the case of coatings on precision instruments.
In order to reduce the incidence of macroparticles contacting the substrate, conventionally a vacuum arc coating apparatus may be constructed with a filtering mechanism that uses electromagnetic fields which direct or deflect the plasma stream. Because macroparticles are neutral, they are not influenced by these electromagnetic fields. Such an apparatus can therefore provide a plasma duct between the cathode chamber and a coating chamber, wherein the substrate holder is installed off of the optical axis of the plasma source. Focusing and deflecting electromagnets around the apparatus thus direct the plasma stream towards the substrate, while the macroparticles, uninfluenced by the electromagnets, would continue to travel in a straight line from the cathode. An example of such an apparatus is described and illustrated in U.S. Pat. No. 5,435,900 issued Jul. 25, 1995 to Gorokhovsky for an xe2x80x9cApparatus for Application of Coatings in Vacuumxe2x80x9d, which is incorporated herein by reference.
Another such apparatus is described in the article xe2x80x9cProperties of Tetrahedral Amorphous Carbon Prepared by Vacuum Arc Depositionxe2x80x9d, Diamond and Related Materials published in the United States by D. R. McKenzie in 1991 (pages 51 through 59). This apparatus consists of a plasma duct made as a quarter section of a tore surrounded by a magnetic system that directs the plasma stream. The plasma duct communicates with two chambers, one chamber which accommodates a plasma source and a coating chamber which accommodates a substrate holder.
The configuration of this apparatus limits the dimensions of the substrate to be coated to 200 mm, which significantly limits the range of its application. Furthermore, there is no provision in the tore-shaped plasma duct for changing the configuration of the magnetic field, other than the magnetic field intensity. Empirically, in such an apparatus the maximum value of the ionic current at the exit of the plasma duct cannot exceed 1 percent of the arc current. This is related to the turbulence of the plasma stream in the tore, which causes a drastic rise in the diffusion losses of ions on the tore walls.
The apparatus taught by Gorokhovsky in U.S. Pat. No. 5,435,900 incorporates a plasma duct surrounded by the deflecting magnetic system, a plasma source and a substrate holder mounted in the coating chamber off of the optical axis of the plasma source, where the plasma source and the substrate holder are surrounded by the focusing electromagnets. The plasma duct is designed in the form of a parallelepiped with the substrate holder and the plasma source mounted on adjacent planes. The magnetic system that forces the plasma stream towards the substrate consists of linear conductors arranged along the edges of the parallelepiped. The plasma duct has plates with diaphragm filters connected to the positive pole of the current source and mounted on one or more planes (not occupied by the plasma source) of the plasma duct. These plates serve as deflecting electrodes to establish an electric field in a transversal direction relative to the magnetic field lines, to guide plasma flow toward the substrate to be coated.
The advantages provided by U.S. Pat. No. 5,435,900 to Gorokhovsky include increasing the range of dimensions of articles (substrates) which can be coated, and providing the user with the option of changing the configuration of the magnetic field in order to increase ionic current at the exit of the plasma duct to 2 to 3 percent of the arc current.
A deflecting electrode is also described in U.S. Pat. No. 5,840,163 issued Nov. 24, 1998 to Welty, which is incorporated herein by reference. This patent teaches a rectangular vacuum arc plasma source and associated apparatus in which a deflecting electrode is mounted inside the plasma duct and either electrically floating or biased positively with respect to the anode. However, this device requires a sensor, which switches the polarity of the magnetic field when the arc spot on the rectangular source has reached the end of the cathode, in order to move the arc spot to the other side of the cathode. This results in an undesirable period where the magnetic field is zero; the arc is therefore not continuous, and is not controlled during this period. This xe2x80x98psuedo-randomxe2x80x99 steering method cannot consistently produce reliable or reproducible coatings.
If the potential of the deflecting electrode (Vd) located opposite the plasma source is greater than the potential of the plasma source wall (Vw), an electric field occurs between them. The intensity of the electric field is given by:                     E        ∝                                            V              d                        -                          V              w                                d                ∝                              σ            ⁡                          [                              1                +                                                      (                                                                  ω                        e                                            ⁢                                              τ                        e                                                              )                                    2                                            ]                                ⁢                      I            d                                              (        1        )            
where
d is the distance between the plate and the plasma duct wall,
xcfx89e is the gyro frequency of magnetized plasma electrons,
xcfx84e is the characteristic time between electron collisions,
"sgr" is the specific resistivity of the plasma in the absence of a magnetic field, and
Id is the current of the deflecting electrode.
Because xcfx89e is proportional to the plasma-guiding magnetic field B, (i.e. xcfx89e xe2x88x9d B), the transversal electric field Et as determined by formula (1) will be proportional to B2, as shown by the following equation:
Etxe2x88x9d"sgr"[1+(xcfx89excfx84e)2]Idxe2x88x9dBt2Idxe2x80x83xe2x80x83(2) 
where Bt is the component of the magnetic field which is tangential to the surface of the deflecting electrode.
An ion is influenced by the force:
Fi=Qixc3x97Eixe2x80x83xe2x80x83(3) 
where Qi is the ion charge. Combining formulae (2) and (3) yields:
Fixe2x88x9dQiBt2Idxe2x80x83xe2x80x83(4) 
This force causes the ion to turn away from the wall opposite the plasma source and directs it towards the substrate to be coated.
In the prior art, most of the surface of the deflecting electrode is disposed in a position where the transversal component of the magnetic field is strong and the tangential component of the magnetic field is relatively weak, which results in low magnetic insulation along the deflecting electrode. This is a disadvantage of the systems taught by Gorokhovsky and Welty, as it results in a weak deflecting electric field which is not strong enough to change the trajectory of heavy metal ions, such as Gf+ and W+, toward the substrate to be coated. Even in the case lighter ions such as Al+ and Ti+ the degree of ion deflection is slight, which results in substantial losses of metal ions before the plasma reaches the position of the substrate(s).
Another method used to reduce the incidence of macroparticles reaching the substrate is a mechanical filter consisting of a baffle, or set of baffles, interposed between the plasma source and the plasma duct and/or between the plasma duct and the substrate. Filters taught by the prior art consist of simple stationary baffles of fixed dimension, such as is described in U.S. Pat. No. 5,279,723 issued Jan. 18, 1994 to Falabella et al., which is incorporated herein by reference. Such filters create large plasma losses and a very low plasma yield, because the baffles destroy the geometry of the plasma duct.
Other mechanical filtering mechanisms, such as that taught by U.S. Pat. No. 5,435,900 to Gorokhovsky, trap macroparticles by altering the path of the plasma stream off of the optical axis of the plasma source toward the substrate, and trapping macroparticles in a baffle disposed generally along the optical axis of the cathode. However, this solution affects macroparticles only and does not allow for control of the plasma composition in the coating chamber, for example where it would be desirable to expose the substrate(s) to an ionized plasma without a metal component, as in plasma immersed processes such as ion implantation, ion cleaning, ion nitriding and the like. As such, prior art vacuum coating apparatus is suitable for use only in plasma vapor deposition (PVD) processes and a separate apparatus is required for plasma immersed processes.
The invention overcomes these disadvantages by providing mechanisms for the effective deflection of the plasma flow, and for controlling the composition of the plasma to allow the apparatus to be used for arc processes other than PVD coating.
In one embodiment the invention provides a coating chamber disposed off of the optical axis of a filtered arc source containing a cathode, wherein an isolated repelling electrode is positioned in the plasma duct, separate from the deflecting electrode, such that the deflecting magnetic field is substantially tangential to a substantial portion of the surface of the repelling electrode. The current applied to the isolated repelling electrode can be varied independently of the current applied to the deflecting electrode, thus allowing the electric field about the repelling electrode to be enhanced and facilitating the function of sustaining the arc in the plasma duct, without altering the deflecting properties of the deflecting electrode
In a further embodiment the repelling electrode is disposed within the plasma duct in the path of the plasma stream, the placement and orientation of the repelling electrode thereby creating an electric field which divides the electric current, physically dividing the plasma stream, which merges after passing around the repelling electrode. The invention thus reduces the loss of metal ions at the substrate in a vacuum arc coating apparatus, and improves the quality of the vacuum within the apparatus. The dividing electrode, being electrically isolated and independently energized, further serves as an auxiliary anode for sustaining the arc in the plasma duct. This embodiment of the invention is particularly advantageously implemented in a vacuum arc coating apparatus in which two plasma sources disposed on opposite sides of a common plasma duct each generate a plasma stream which combine at the entrance to the plasma duct and flow into the coating chamber.
The repelling electrode and/or the deflecting electrode can be maintained at floating potential, or can be connected to the positive poles of separate power sources so that the applied current can be varied independently of one another.
In a further embodiment the deflecting electrode is surrounded by a baffle for removing macroparticles from the plasma stream, which also serves as a getter pump to remove gaseous contaminants from within the apparatus. When the baffle is maintained at floating or negative potential, ions are adsorbed to the surface of the baffle.
The invention also provides a multiple-cathode apparatus suitable for use in plasma immersed processes as ion implantation, ion nitriding, ion cleaning and the like. In these embodiments a first filtered arc source containing one or more cathodes generates cathodic evaporate for coating the substrate, while the deflecting and focusing magnetic fields affecting a second filtered arc source are deactivated so that cathodic evaporate does not flow toward the substrates. The second filtered arc source thus functions as a powerful electron emitter for plasma immersed treatment of the substrates.
Optionally in these embodiments a load lock shutter comprising a metallic grid is disposed between the plasma duct and the coating chamber, to control communication between the plasma source and the coating chamber. Where particularly contaminant-free conditions are required the load lock shutter can be closed to contain macroparticles and metal vapour within the cathode chamber(s) and plasma duct, but permit the passage of electrons into the coating chamber to thus increase the ionization level of a gaseous component within the coating chamber. The load lock shutter can be charged with a negative potential, to thus serve as an electron accelerator and ion extractor. Optionally load lock shutters may also be provided between the filtered arc source and the plasma duct, and/or between the cathodes and the deflecting electrode within a filtered arc source.
The load lock shutters can also be used in conjunction with the deflecting electrode operating as a getter pump, to improve the quality of the vacuum in the chamber.
The present invention thus provides an apparatus for the application of coatings in a vacuum, comprising at least one filtered arc source comprising at least one cathode contained within a cathode chamber, at least one anode associated with the cathode for generating an arc discharge, a plasma duct in communication with the cathode chamber and with a coating chamber containing a substrate holder for mounting substrates to be coated, the substrate holder being positioned off of an optical axis of the cathode, at least one deflecting electrode electrically insulated from the plasma duct and disposed adjacent to one or more walls of the plasma duct that are not occupied by the cathode, at least one deflecting conductor disposed adjacent to the plasma source and the plasma duct, and at least one repelling electrode connected to the positive pole of a current source and disposed along the plasma duct at a position between the deflecting electrode and the coating chamber.
In an embodiment in which a pair of cathodes are disposed in a filtered arc source on opposite sides of the plasma guide, at least a portion of the deflecting electrode and the repelling electrode are disposed in alignment with a plane of symmetry between opposite walls of the plasma guide. The plane of symmetry extends between magnetic cusps generated by deflecting conductors disposed adjacent to the plasma duct at the intersection with the filtered arc source, and the repelling electrode can thus be positioned between the cusps in a portion of the plasma duct in which a tangential component of the deflecting magnetic fields is strongest.
In a further aspect of the invention the apparatus comprises at least one focusing conductor positioned adjacent to the plasma duct between the deflecting conductor and the coating chamber for generating a focusing magnetic field which focuses plasma entering the coating chamber.
In a further aspect of the invention the repelling electrode is disposed near a position where a tangential component of a magnetic field within the plasma duct is strongest.
The present invention further provides an apparatus for the application of coatings in a vacuum, comprising a plurality of substantially opposed cathode chamber pairs, each cathode chamber containing a cathodic arc source and being disposed along a plasma duct in communication with each of the cathode chambers and in communication with a coating chamber containing a substrate holder for mounting substrates to be coated, the substrate holder being positioned off of an optical axis of the cathodic arc sources, at least one anode associated with each cathodic arc source for generating an arc discharge, a deflecting system for deflecting a flow of plasma through the plasma duct toward the substrate chamber, and a plurality of magnetic isolating coils each disposed about the plasma duct between cathode chamber pairs, transversely relative to the plasma duct and relative to a direction of the cathodic arc flow through the plasma duct, wherein when an isolating coil is activated a flow of plasma is confined by the isolating coil.
The present invention further provides a method of coating an article in a coating apparatus comprising a plurality of substantially opposed cathode chambers each supporting a cathodic arc source and being disposed along an elongated plasma duct in communication with the cathode chambers, at least one anode associated with each cathodic arc source, a plurality of magnetic isolating coils each disposed transversely relative to the plasma duct between cathode chamber pairs, and a coating chamber in communication with an end of the plasma duct, the method comprising the steps of: a) generating an arc between the cathodic arc source and its associated anode to create a plasma of cathodic evaporate, and b) selectively activating the isolating coils to confine the plasma within a cell formed between isolating coils for a selected interval.
In a further embodiment the isolating coils and the cathodic arc sources are energized sequentially, to raster the plasma jets along the plasma duct. When the impulse cathodic arc sources are energized plasma jets are generated along magnetic field lines created by the deflecting magnetic system and a rastering system comprising the isolating coils. These magnetic coils combine to create a magnetic field which guides the plasma jets toward the coating chamber. By rastering the magnetic fields in conjunction with impulsing the cathodic arc sources, each impulse provides a portion of metal plasma to a different location along the substrates.
The present invention further provides an apparatus for the application of coatings in a vacuum, comprising at least one plasma source comprising a cathode contained within a cathode chamber, at least one proximal anode associated with the cathode for generating an arc discharge, a plasma duct in communication with the cathode chamber and with a coating chamber containing a substrate holder for mounting substrates to be coated, the substrate holder being positioned off of an optical axis of the cathode, and at least one auxiliary anode disposed downstream of the plasma source for generating an auxiliary arc discharge.
The present invention further provides an apparatus for the application of coatings in a plasma-immersed environment, comprising a first plasma source comprising a first cathode contained within a cathode chamber and associated with an anode for generating an arc discharge between the anode and the first cathode, a plasma duct in communication with the cathode chamber and with a coating chamber containing a substrate holder for mounting substrates to be coated, the substrate holder being positioned off of an optical axis of the first cathode, a second plasma source comprising a second cathode in communication with the coating chamber and associated with an anode for generating an arc discharge between the anode and the second cathode, a deflecting system for directing a flow of plasma to the coating chamber, wherein the deflecting system can be deactivated while the first plasma source is activated so that plasma from the first cathode does not flow into the coating chamber but electrons emitted from the first cathode flow into the coating chamber.
The present invention further provides a method of coating a substrate in a plasma-immersed environment, comprising the steps of a). activating a first plasma source comprising a first cathode contained within a first cathode chamber in communication with a coating chamber containing a substrate holder, the substrate holder being positioned off of an optical axis of the first cathode, b). activating a second plasma source comprising a second cathode contained within a second cathode chamber in communication with the coating chamber, at a position remote from the first cathode chamber and c). selectively deactivating a deflecting system that directs plasma from the first cathode into the coating chamber and establishing an auxiliary arc discharge between the first cathode and an anode contained within the coating chamber, so that ions emitted from the first cathode are substantially prevented from flowing into the coating chamber.
The present invention further provides an apparatus for the application of coatings in a vacuum, comprising at least one filtered arc source comprising at least one cathode contained within a cathode chamber, at least one anode associated with the cathode for generating an arc discharge, at least one auxiliary anode disposed downstream of the plasma source for generating an auxiliary arc discharge a plasma duct in communication with the cathode chamber and with a coating chamber containing a substrate holder for mounting substrates to be coated, the substrate holder being positioned off of an optical axis of the cathode, at least one deflecting electrode electrically insulated from the plasma duct and disposed adjacent to one or more walls of the plasma duct that are not occupied by the cathode, at least one deflecting conductor disposed adjacent to the plasma source and the plasma duct, and at least one metal vapor plasma source disposed opposite to substrate holder, comprising crucible containing material to be evaporated, the crucible being shielded from a surrounding plasma environment.